Process for passivating pyrophorous catalysts

ABSTRACT

The present invention relates to a process for passivating pyrophorous solids by using nitrogen, carbon dioxide and oxygen under closely defined conditions to obtain a uniformly passivated solid.

DESCRIPTION

The present invention relates to a process for passivating pyrophoroussolids, especially catalysts, and the thus prepared solids themselves.

The passivation of supported nickel catalysts is known. Existingprocesses rely in many cases on a partial oxidation of the metalsurface. DE-B-1 299 286, DE-A-2 209 000 and DE-A-2 530 818 describepassivation processes in aqueous solution. The oxidizing agents used arehydrogen peroxide, hypochlorites or oxygen. The processes described,however, are not universally useful, since there are many applicationswhere the water has to be removed before use of the catalyst.

U.S. Pat. No. 2,565,347, U.S. Pat. No. 3,868,332, DD 156 169, DD 156345, DD 156 347, DD 157 161 and RO 68 600 disclose the passivation ofpyrophorous nickel-containing catalysts with oxygen-containing nitrogen.In the processes described, the catalyst is purged with nitrogen priorto passivation to desorb hydrogen from the catalyst surface. Thedisclosed processes differ from each other with regard to thetemperature settings for the desorption and the passivation. One of thedisadvantages of the processes described is that they provide, whenpracticed on an industrial scale, non-uniform products and require longpassivation times.

EP-A 89 761 describes the passivation of a nickel-Al₂O₃ catalyst bytreating the catalyst with CO₂ at temperatures of 175 to 200° C. for aperiod of at least 30 minutes and subsequent cooling in CO₂ to ambienttemperature.

U.S. Pat. No. 4,090,980 discloses the passivation of a catalyst with anoxygen-nitrogen mixture after prior treatment with CO₂, which comprisestreating the reduced catalyst with an inert gas by circulating the inertgas through it at a catalyst temperature of 149° C., then cooling thecatalyst to a temperature of 10 to 38° C., then progressively increasingthe CO₂ content in the circulating gas to 80% by volume and subsequentlyadding oxygen to obtain a concentration of 0.05% by volume. Thetreatment with this mixture is carried on until 25% of the monolayer ofthe catalyst surface is coated with oxygen. The oxygen concentration isthen raised to 1% by volume and, after the monolayer on the catalystsurface has been completely formed by oxygen species, the oxygen contentis slowly raised and the CO₂ content of the gas mixture reduced.

A combined CO₂ and O₂ treatment is also disclosed in SU 1 344 404. Inthis passivation process, the catalyst is treated with CO₂ attemperatures of 250 to 300° C., subsequently in the CO₂—O₂ mixture withoxygen concentrations of 0.4 to 2% by volume at temperatures of 100 to250° C. for a period of 20 minutes, the catalyst is cooled down toambient temperature under CO₂ and subsequently air is flowed through thecatalyst bed.

DE 3 629 631 describes the passivation of nickel-containing catalystswith CO₂, steam or oxygen. When the passivation has been carried outwith CO₂ in the temperature range from 25 to 250° C., this disclosurerequires that the CO₂ treatment be followed by a further passivationwith oxygen-nitrogen mixtures.

In summary it can be stated that existing passivation processes requirevery long passivation times, especially to passivate withoxygen-nitrogen mixtures, and so are very costly and, what is more,provide nonuniformly passivated catalysts.

It is an object of the present invention to provide a process forpassivating pyrophorous solids, especially pyrophorous catalysts, thatleads to more uniformly passivated solids, especially catalysts, at lowcost.

This object is achieved by a process for passivating a preferablyreduced and/or preferably inertized pyrophorous solid, especially acatalyst, particularly preferably a supported metal catalyst, whereinthe catalyst a) is treated in a CO₂—N₂ gas mixture having a CO₂ contentof 0.5 to 10% by volume at temperatures of 91° C. to 350° C. for atleast 30 minutes, b) then cooled down to a temperature of not more than90° C. in the CO₂—N₂ gas mixture of step a), c) after reaching thetemperature mentioned in step b) oxygen, preferably air, is added to theCO₂—N₂ gas mixture in a first passivation phase up to a content of 0.2to 1.5% by volume and the catalyst is treated in the CO₂—N₂—O₂ gasmixture for at least 30 minutes by shaking, d) and then in a secondpassivation phase, with shaking, the CO₂ content in the CO₂—N₂—O₂ gasmixture is lowered to <0.1% by volume and the O₂ content is raised to1.5—21% by volume.

The process of the invention has the advantage of short stabilizationtimes while at the same time providing readily reactivable catalystspossessing very good thermal stability. Advantageously, the catalystsare passivated particularly uniformly. It is in fact surprising that thetreatment with low-CO₂ inert gases under the stated conditions providesvery uniformly and easily reactivable catalysts.

For the purposes of the present invention, a pyrophorous solid is asolid body that ignites, or tends to ignite, spontaneously, especially abody that will spontaneously combust in air when exposed to it in astate of very fine subdivision.

In a particularly preferred embodiment, the invention provides anaforementioned process wherein the pyrophorous solid is a pyrophoroussupported metal catalyst. The invention accordingly provides for examplesupported metal catalysts wherein the metal component is nickel, cobalt,copper, iron, aluminum, zinc or mixtures or alloys of two or morethereof. In a preferred embodiment, the support component of thesupported metal catalyst used according to the invention consists of orcontains for example Al₂O₃, SiO₂, SiO₂ ·Al₂O₃, TiO₂, ZrO₂, mixtures ofthese oxides, activated carbon, zeolites, clays, natural silicates ormixtures of two or more thereof. The invention also provides that thesupports may have been chemically modified, for example by treatmentwith phosphate, sulfate, fluoride or chloride compounds. It will beappreciated that, according to the invention, it is also possible forthe pyrophorous supported metal catalyst used to be doped, for exampleby additions of elements of the sixth to eight transition group of thePeriodic Table of the Elements, such as platinum, palladium, rhodium,chromium, tantalum, titanium, iron or their mixtures etc.

It will be appreciated that the supported metal catalyst used accordingto the invention can additionally a contain additive materials such asmolding assistants, lubricants, plasticizers, pore-formers, moisteners,etc.

The herein described process for passivating a solid starts with areduced and inertized solid. The solid, especially a catalyst, can bereduced by treating the catalyst in a hydrogen stream at elevatedtemperatures. After reduction, the catalyst is, if appropriate,inertized in a nitrogen stream. The invention thus provides in apreferred embodiment that the solid, especially a catalyst, to bepassivated is reduced and/or inertized before passivation. The reducingcan be effected by treating the solid with a hydrogen stream attemperatures of 250 to 500° C., a volume hourly space velocity of 250 to300 v/v h and a heating rate of 50° C./h to 200° C./h, preferably atperiods of 2 hours to 16 hours at reduction temperature. This can befollowed by an inertization under the following conditions: at reductiontemperature the hydrogen stream is switched over to a nitrogen stream,inertization is carried out at this temperature for about 30 minutes andthis is followed by cooling down in the nitrogen stream to thetemperature of the treatment with the N₂—CO₂ mixture, the N₂ volumehourly space velocity being 250 to 3 000 v/v h.

The oxygen to be added during the first passivation phase of step c) canpreferably be added by adding air up to the stated oxygen concentration.It will be appreciated, however, that the oxygen can also be added inpure form for example.

In a preferred embodiment of an aforementioned process providedaccording to the invention, the process according to the invention iscontinuously or batch operated in a catalyst bed, especially in acatalyst bed whose height to diameter ratio is in the range from 0.05 to1.

In a further preferred embodiment, the concentration of the CO₂ duringthe treatment with the CO₂—N₂ mixture as per the first step a) is 1 to2.5% by volume.

In a further preferred embodiment, the volume hourly space velocityduring the treatment with the CO₂—N₂ mixture as per the first step a) is500 to 10 000 v/v h. In a further preferred embodiment, the volumehourly space velocity during the treatment with the CO₂—N₂ mixture asper the first step a) and/or during the treatment with the CO₂—N₂—O₂ gasmixture as per the third and fourth steps c) and d) is 100 to 3 000 v/vh.

In a further preferred embodiment, the treatment in the CO₂—N₂—O₂ gasmixture as per the third and fourth steps c) and d) is carried out for aperiod of more than 30 minutes, for example 33 minutes to 8 hours. Stepd) can be carried out for a period of not less than 3 minutes, in apreferred embodiment.

In a further development of the aforementioned process, the ratio of theduration of the treatment as per the third step c), i.e., the firstpassivation phase, to the duration of the fourth step d), i.e., thesecond passivation phase, is 9:1.

In a further preferred embodiment, the temperature of the treatment ofthe catalyst with the CO₂—N₂—O₂ gas mixture of step c) and/or d) is 50to 70° C.

In a further preferred embodiment, the CO₂ concentration in theCO₂—N₂—O₂ gas mixture during the treatment of the third step c) is 0.5to 1.5% by volume. In a preferred embodiment, the CO₂ content of themixture from step a) can be lowered, for example to the aforementionedrange, for the duration of step c).

In a further preferred embodiment of the present invention, there isprovided an aforementioned process wherein the O₂ concentration in theCO₂—N₂—O₂ gas mixture during the treatment of the third step c) is 0.25to 0.8% by volume.

In a further preferred embodiment of the invention, the O₂ concentrationduring the treatment of the fourth step d) is 5 to 10% by volume.

In a further embodiment, the invention provides an aforementionedprocess wherein the shaking of the catalyst bed during steps c) and/ord) is effected at intervals of 10 to 20 minutes for a period of 0.5 to 2minutes in each case. It is advantageous to employ shaking frequenciesof 10 to 50 Hz.

It will be appreciated that it is also possible, especially in the caseof pulverulent catalysts and catalysts possessing very high strengths,to agitate the catalyst bed by fluidization or disposition in a rotarytube oven. At any rate, it is an essential aspect of the presentinvention to agitate the catalyst in the oxygen-carbon dioxide-nitrogenmixture at least temporarily during the passivation phases of steps c)and d), for example in a moving bed.

The invention also provides a passivated solid, especially a passivatedsupported metal catalyst, prepared according to one of the subjectprocesses. Such catalysts are notable for good reactivability, excellentthermal air stability and uniform passivation.

The examples hereinbelow illustrate the invention.

EXAMPLES

The inventive and comparative examples were carried out using an Ni-SiO₂catalyst (2 mm extrudates) having an Ni content of 62.6% by mass, whichprior to passivation was reduced by the following process: 25 1 of thecatalyst were subjected to a hydrogen stream at a volume hourly spacevelocity of 1 250 v/v h while being heated to 445° C. at a rate of 50°C./h and reduced at 445° C. for 5 hours. The reduced catalyst has an Nisurface area of 42.9 m²/g_(cat).

Example 1 (Inventive)

The passivation is carried out in a reactor which is equipped with ashaking apparatus and in which the height to diameter ratio of thecatalyst bed is 0.2. After reduction, the catalyst is cooled down to300° C. in a nitrogen stream at a volume hourly space velocity of 2 000v/v h. At 300° C., CO₂ is added to the nitrogen until a concentration of1.5% by volume is measured in the circulating gas. The catalyst is thencooled down to 60° C. in this gas mixture in the course of 1 hour. Afterattaining 60° C., air is added to an O₂ content of 0.25% by volume, theCO₂ content in the circulating gas lowered to 0.8% by volume and thecatalyst treated with this gas mixture for 2.5 hours. During thistreatment at 60° C., the catalyst bed is shaken at a frequency of 35 Hzfor a period of 40 seconds at intervals of 15 minutes. The O₂ content isthen raised to 5% by volume, the CO₂ content lowered to 0.05% by volumeand the catalyst treated in this gas mixture for 15 minutes. The totalstabilization time is about 4.5 hours.

Example 2

The passivation is carried out in the same reactor as in inventiveexample 1. After reduction, the catalyst is cooled down to 180° C. in anitrogen stream at a volume hourly space velocity of 1 500 v/v h. At180° C., CO₂ is added to the nitrogen until a concentration of 2.5% byvolume is measured in the circulating gas. The catalyst is treated withthe CO₂—N₂ mixture at 180° C. for 30 minutes and then cooled down to 50°C. in this gas mixture in the course of 0.5 hours and, after air isadded the catalyst is stabilized for 3 hours at this temperature with acirculating O₂—CO₂—N₂ mixture having a CO₂ content of 1.8% by volume andan O₂ content of 0.5% by volume. During this treatment at 50° C., thecatalyst bed is shaken at a frequency of 35 Hz for a period of 40seconds at intervals of 10 minutes. The O₂ content is then raised to 8%by volume, the CO₂ content lowered to 0.05% by volume and the catalysttreated in this gas mixture for 20 minutes. The total stabilization timeis about 5 hours.

Example 3 (Comparative)

The stabilization was carried out in a tubular oven having a height todiameter ratio of 8 for the catalyst bed. After reduction, the catalystis cooled down to 40° C. in a nitrogen stream at a volume hourly spacevelocity of 1 500 v/v h. At 40° C., O₂ is added to the nitrogen until aconcentration of 0.3% by volume is measured in the circulating gas. Thecatalyst is then treated in this O₂—N₂ mixture for 20 hours. Finally theO₂ content is raised to 8% by volume and the catalyst is treated in thisgas mixture for 50 minutes. The total stabilization time is about 22hours.

Example 4 (Comparative)

After reduction, the reduced catalyst is cooled down to room temperaturein a nitrogen stream, then treated with pure CO₂ at a volume hourlyspace velocity of 1 000 v/v h for a period of 2 hours and finally air ispassed through the sample for a period of 19 hours, the air rate chosenbeing such that the temperature in the catalyst bed does not exceed 80°C. The total stabilization time is about 21 hours.

Example 5 Comparison of Catalysts with Regard to Their Reactivability,Thermal Air Stability and Catalytic Activity

The catalysts were characterized by TPR. The TPR investigations werecarried out with an Ar—H₂ mixture containing 10% by volume of H₂; theheating rate was 10° C./minute. The position of the peak in the TPRspectrum was taken as a measure of the reactivability of the passivatedcatalyst: the lower the position of the peak, the easier thereactivability of the catalyst. The results of the individual examplesare summarized in the table.

The thermal air stability of the catalysts was determined as well. Tothis end, 20 g of each catalyst were heated in air at a rate of 10°C./minute while the temperature in the catalyst bed was monitored.Thermal stability was taken to be measured by the light-off temperature.The light-off temperature is the temperature at which the catalyststarts to burn, which is detectable from a very pronounced temperaturerise in the catalyst bed. Light-off temperatures of above 90° C. aredesirable for convenient catalyst handling.

The catalytic characteristics of the catalysts were determined in thebenzene hydrogenation test: 100 mg of the catalyst were subjected to ahydrogen stream of 3l/h while being heated to 100° C. at a rate of 5°C./minute and maintained at 100° C. for 1 hour for reactivation. Ahydrogen-benzene mixture (5l of H₂/h, 0.5 ml of benzene/h) is thenpassed through the catalyst bed. After one hour, the reaction mixture isanalyzed by gas chromatography. The degree of conversion of benzene intocyclohexane is a measure of the catalytic activity. The results arelikewise summarized in the table.

TABLE Temperature of Lightoff Benzene peak in TPR temperature conversionCatalyst of curve in ° C. in ° C. in % Example 1  95 120 36 (inventive)Example 2  90 123 38 (inventive) Example 3 184  80 11 (comparative)Example 4 191 197  6 (comparative)

The results are clear in showing the advantages of the process accordingto the invention. It provides very good thermal air stability for thecatalysts coupled with short stabilization times and goodreactivability.

What is claimed is:
 1. A process for passivating a pyrophorous solid,wherein the solid a) is treated in a CO₂—N₂ gas mixture having a CO₂content of 0.5 to 10% by volume at temperatures of 91° C. to 350° C. forat least 30 minutes, b) then cooled down to a temperature of not morethan 90° C. in the CO₂—N₂ gas mixture of step a), c) after reaching thetemperature of step b) oxygen is added to the CO₂—N₂ gas mixture in afirst passivation phase to a content of 0.2 to 1.5% by volume and thecatalyst is treated in the CO₂—N₂—O₂ gas mixture for at least 30 minuteswith agitation, d) and then in a second passivation phase, withagitation, the CO₂ content in the CO₂—N₂—O₂ gas mixture is lowered to<0.1% by volume and the oxygen content is raised to 1.5—21% by volume.2. A process as claimed in claim 1, wherein the CO₂ content during thetreatment with the CO₂—N₂ gas mixture of step a) is 1 to 2.5% by volume.3. A process as claimed in claim 1, wherein the O₂ content in theCO₂—N₂—O₂ gas mixture is 0.25 to 0.8% by volume during the treatment ofstep c) or 5 to 10% by volume during step d), or both.
 4. A process asclaimed in claim 1, wherein the solid is a catalytic solid.
 5. A processas claimed in claim 4, wherein the solid is a metal-containing catalyst.6. A process as claimed in claim 5, wherein the solid is reduced priorto step c).
 7. A process as claimed in claim 6, wherein the solid isrendered inert prior to passivation and after any reduction.
 8. Aprocess as claimed in claim 5, wherein the metal is nickel, cobalt,copper, iron, aluminum, zinc or a mixture or an alloy thereof.
 9. Aprocess as claimed in claim 5, wherein the metal is on a support ofSiO₂, Al₂O₃, SiO₂·Al₂O₃, ZrO₂, TiO₂, clay, zeolite, activated carbon,natural silicate or a mixture thereof.
 10. A process as claimed in claim5, wherein the process is conducted in a catalyst bed.
 11. A process asclaimed in claim 5, wherein the CO₂ content during the treatment withthe CO₂—N₂ gas mixture of step a) is 1 to 2.5% by volume.
 12. A processas claimed in claim 5, wherein the volume hourly space velocity duringthe treatment with the CO₂—N₂ gas mixture of step a) is 500 to 10,000v/v h.
 13. A process as claimed in claim 12, wherein the volume hourlyspace velocity during the treatment with the CO₂—N₂ gas mixture of stepa) or the CO₂—N₂—O₂ gas mixture of step c) or step d) is 1,000 to 3,000v/v h.
 14. A process as claimed in claim 5, wherein the catalyst istreated in the CO₂—N₂—O₂ gas mixture of steps c) and d) for longer than0.5 hours.
 15. A process as claimed in claim 5, wherein the temperatureof the treatment of the catalyst with the CO₂—N₂—O₂ gas mixture of stepc) and step d) is 50 to 70° C.
 16. A process as claimed in claim 5,wherein the CO₂ content in the CO₂—N₂—O₂ gas mixture during thetreatment of step c) is 0.5 to 1.5% by volume.
 17. A process as claimedin claim 5, wherein the durations of the treatment in steps c) to d) arein a ratio of 9:1.
 18. A process as claimed in claim 5, wherein the O₂content in the CO₂—N₂—O₂ gas mixture is 0.25 to 0.8% by volume duringthe treatment of step c) or 5 to 10% by volume during step d), or both.19. A process as claimed in claim 5, wherein the agitation of thecatalyst bed during steps c) or d), or both, is effected at intervals of10 to 20 minutes for a period of 0.5 to 2 minutes in each case.
 20. Aprocess as claimed in claim 19, wherein the process is conducted in abed of catalyst which is a pyrophororus metal selected from the groupconsisting of nickel, cobalt, copper, iron, aluminum, zinc or a mixtureor an alloy thereof on a support of SiO₂, Al₂O₃, SiO₂·Al₂O₃, ZrO₂, TiO₂,clay, zeolite, activated carbon, natural silicate or a mixture thereof;the CO₂ content during the treatment with the CO₂—N₂ gas mixture is 1 to2.5% by volume and the volume hourly space velocity is 500 to 10,000 v/vh during the treatment with the CO₂—N₂ gas mixture of step a); thecatalyst is treated in the CO₂—N₂—O₂ gas mixture of steps c) and d) at atemperature of 50 to 70° C. and for longer than 0.5 hours; the O₂content in the C_(O) ₂—N₂—O₂ gas mixture is 0.25 to 0.8% by volumeduring the treatment of step c) or 5 to 10% by volume during step d), orboth; and wherein the agitation of the catalyst bed during steps c) ord), or both, is effected at intervals of 10 to 20 minutes for a periodof 0.5 to 2 minutes in each case.
 21. A process as claimed in claim 20,wherein the process is conducted in a catalyst bed whose height todiameter ratio is in the range from 0.05 to 1; the volume hourly spacevelocity during the treatment with the CO₂—N₂ gas mixture of step a) orthe CO₂—N₂—O₂ gas mixture of step c) or step d) is 1,000 to 3,000 v/v h;the catalyst is treated in the CO₂—N₂—O₂ gas mixture of steps c) and d)for 33 minutes to 8 hours and the durations of the treatment in steps c)to d) are in a ratio of 9:1; and wherein the CO₂ content in theCO₂—N₂—O₂ gas mixture during the treatment of step c) is 0.5 to 1.5% byvolume.
 22. A process as claimed in claim 21, wherein the solid isreduced and then rendered inert prior to step c).
 23. A passivatedsupported metal catalyst prepared by the process of claim 1.